1. Field of the Invention
The invention relates to methods and apparatus for testing video equipment and a video test signal for use therewith.
2. Description of the Prior Art
As with all other types of technology, it is desirable to test the performance, both during manufacture and during operation, of video equipment. In the case of equipment for use in broadcast and professional applications, a series of rigorous tests have been established. For example, a number of standard full-field video test signals need to be applied in order to fully test the equipment. The conventional full-field test signals take the form of full-field images. Examples of such images are vertical colour bar images, pulse and bar images, pedestal (grey scale) images, modulated staircase (e.g. 5-step) images, and multiburst (frequency response) images.
Traditionally, the application of the various test signals was a time consuming and labour intensive process as this was performed manually. In simple terms, the traditional process comprised storing a test signal on tape followed by replaying and analysing the recorded test signal, then repeating these steps for each of the test signals to be applied. To avoid having to repeatedly sequence through the stages of record, rewind and playback, an alternative approach was to store a series of test signals on the tape, rewind to the beginning and then playback once through all the separate test signals.
More recently, however, it has become common practice to employ automatic analysers to analyse the test signals output from the tape in order to enable more objective quantitative measurements of the performance of the equipment. Typically, when automatic analysis is employed, each test signal is taken in turn. In other words, a test signal is recorded on tape, the tape is then rewound, replayed and analysed. These steps are then repeated for the second test signal to be applied, the third test signal to be applied, and so on. With existing technology it is possible to control conventional signal generators to automatically generate the series of test signals to control the automatic analysers to analyse the series of test signals and to control the recording, rewinding and replay modes of the video tape recorder. However, it will be appreciated that this is still a time consuming process due, not least, to the need repeatedly to switch between the record, rewind and playback modes of the video tape recorder.
The alternative approach used with fully manual testing, namely recording a series of test signals in one go and then analysing the series of test signals, presents severe difficulties due to the need to tightly synchronise the record and playback modes to ensure that the analyser only carries out measurements when the correct signal is being replayed.
Test signal generators have been known for over a decade which generate complex test patterns formed as a matrix of different test signals within one field of a video image. The Tektronix 1411 video generator is able to produce a matrixed test signal which has parts of five of the standard full frame test signals used in the industry. This matrixed test signal, which in practice is in colour, is represented in monochrome in FIG. 1. The matrixed test signal comprises the following test signals: colour bars 12; shallow ramp 14; pulse and bar 16; modulated staircase 18 and multiburst 20.
Another matrixed test signal can be produced by the Tektronix TPG625, Option 01 PAL pattern generator. The full-field matrix composition of this signal includes parts of twelve separate test signals which are reproduced on the top half of the field and repeated in the bottom half of the field.
These matrixed test signals mean that it is possible to apply a number of test signals to a piece of equipment at one time. The matrixed test signals do not, however, attempt or enable full-field analysis over the whole video field in that any one of the test signals is only reproduced in a limited area of the video field. In other words, the matrixed test signals discussed above do not form a satisfactory replacement for the conventional full-field test signals.
In particular, these matrixed test signals are not suitable for testing the operation of video recorders. As known to the skilled person, video tape recorders typically record video information with a rotating head laying a helical track on a video tape. FIG. 2 represents a video tape T with helically recorded tracks t1, t2, t3, t4, t5, etc, thereon. It will be appreciated by the skilled person that in practice the tracks are much more closely spaced than indicated in FIG. 2. It is assumed that each track corresponds to a field of video information with the lines of the field from top to bottom being recorded sequentially from the bottom left to the top right of the track. In order to represent a full interlaced frame of video information, two fields (e.g. t1, t2) are recorded on the tape, typically in successive tracks.
With this technology it is not possible to guarantee an even recording level across the tape due to characteristics of the tape and the recording technology. Typically, stronger signals tends to be recorded near the middle of the tape than at the edges of the tape. In view of these technical limitations, the matrixed signals discussed above are not considered acceptable for fully testing the operation of video tape recorders, particularly high quality video tape recorders for broadcast and professional use. Accordingly, it has been necessary to continue using the traditional full-field test signals.